The present invention pertains to the fields of cemented carbide parts, having cobalt, nickel, iron or their alloys as a binder material, and the manufacture of these parts. More particularly, the present invention pertains to cemented carbide metal cutting inserts having a hard refractory oxide, nitride, boride, or carbide coating on their surface.
In the past, various hard refractory coatings have been applied to the surfaces of cemented carbide cutting inserts to improve the wear resistance of the cutting edge and thereby increase the cutting lifetime of the insert. See, for example, U.S. Pat. Nos. 4,035,541 (assigned to applicant corporation); 3,564,683; 3,616,506; 3,882,581; 3,914,473; 3,736,107; 3,967,035; 3,955,038; 3,836,392; and U.S. Pat. No. 29,420. These refractory coatings, unfortunately, can reduce the toughness of cemented carbide inserts to varying degrees. The degree of degradation depends at least in part on the structure and composition of the coating and the process used for its deposition. Therefore, while refractory coatings have improved the wear resistance of metal cutting inserts, they have not reduced the susceptibility of the cutting edge to failure by chipping or breakage, especially in interrupted cutting applications.
Previous efforts to improve toughness or edge strength in coated cutting inserts revolved around the production of a cobalt enriched layer extending inwardly from the substrate/coating interface. It was found that cobalt enrichment of the surface layers in certain C porosity substrates could be achieved during vacuum sintering cycles. These cobalt enriched zones were characterized by A porosity while most of the bulk of the substrate had C porosity. Solid solution carbide depletion was usually present to varying depths and degrees in the areas of cobalt enrichment. Cobalt enrichment is desirable in that it is well known that increasing cobalt content will increase the toughness or impact resistance of cemented carbides. Unfortunately, the level of enrichment produced is difficult to control in C porosity substrates. Typically, a coating of cobalt and carbon was formed on the surface of the substrate. This coating of cobalt and carbon was removed prior to deposition of the refractory material on the substrate, in order to obtain adherent bonding between the coating and substrate. At times, the level of cobalt enrichment in the layers beneath the surface of the substrate was so high that it had an adverse effect on flank wear. As a result, sometimes the layer of cobalt enrichment on the flank faces of the substrate were ground away leaving cobalt enrichment only on the rake faces and the possibility of C porosity material on the flank face. In comparison with A or B type porosity substrates, C porosity substrates are not as chemically homogeneous. This can result in less control over the formation of eta phase at the coating substrate interface (a hard and brittle phase affecting toughness), a reduction in coating adherency and an increase in nonuniform coating growth.
By way of definition, the porosity observed in cemented carbides may be classified into one of three categories recommended by the ASTM (American Society for Testing and Materials) as follows:
Type A for pore sizes less than 10 microns in diameter.
Type B for pore sizes between 10 microns and 40 microns in diameter.
Type C for irregular pores caused by the presence of carbon inclusions. These inclusions are pulled out of the sample during metallographic preparation leaving the aforementioned irregular pores.
In addition to the above classifications, the porosity observed can be assigned a number ranging from 1 through 6 to indicate the degree or frequency of porosity observed. The method of making these classifications can be found in Cemented Carbides by Dr. P. Schwarzkopf and Dr. R. Kieffer, published by the MacMillan Co., New York (1960) at Pages 116 to 120.
Cemented carbides may also be classified according to their binder carbon and tungsten contents. Tungsten carbide-cobalt alloys having excess carbon are characterized by C porosity which, as already mentioned, are actual free carbon inclusions. Tungsten carbide-cobalt alloys low in carbon and in which the cobalt is saturated with tungsten are characterized by the presence of eta phase, a M.sub.12 C or M.sub.6 C carbide, where M represents cobalt and tungsten. In between the extremes of C porosity and eta phase, there is a region of intermediate binder alloy compositions which contain tungsten and carbon in solution to varying levels, but such that no free carbon or eta phase are present. The tungsten level present in tungsten carbide cobalt alloys can also be characterized by the magnetic saturation of the binder alloy, since the magnetic saturation of the cobalt alloy is a function of its composition. Carbon saturated cobalt is reported to have a magnetic saturation of 158 gauss-cm.sup.3 /gm cobalt and is indicative of C type porosity, while a magnetic saturation of 125 gauss-cm.sup.3 /gm cobalt and below indicates the presence of eta phase.
It is, therefore, an object of the present invention to provide a readily controllable and economic process for producing a binder enriched layer near the surface of a cemented carbide body.
It is a further object of this invention to provide a cemented carbide body having a binder enriched layer near its surface with substantially all porosity throughout the body being of the A or B types.
It is also an object of this invention to provide cemented carbide bodies having carbon levels ranging from C porosity to eta phase with a binder enriched layer near their peripheral surface.
It is an additional object of this invention to combine the aforementioned cemented carbide bodies according to the present invention with a refractory coating so as to provide coated cutting inserts having a combination of high wear resistance and high toughness.
These and other objects of the present invention will become more fully apparent upon review of the following description of the invention.